Hydraulic hold-down



April 3, 1951 F. E. MUNSCHAUER HYDRAULIC HOLD-DOWN 4 Sheets-Sheet 2Filed May 27, 1948 Fig.5

gwg Qfluw ATTORNEYS F. E. MUNSCHAUE HYDRAULIC HOLD-DOWN 'Aprifi 3, E91

' Filed May 27, 1948 4 Sheets-Sheet 4 INVENTOR M3 M6 fieder/c/f E. Mwzq/zauer ATTORNEYJ Patented Apr. 3, 1951 antes HYDRAULIC HOLD-DOWNFrederick E. Munschauer, Buffalo, N. Y., assignor to Niagara lilachine &Tool Works, Bufialo,

Application May 27, 1948, Serial No. 29,508

(Cl. 16i-52 20 Claims. 1

This invention relates to hydraulic hold-down means for power shears andanalogous machines. Conventional hold-down means for holding metal orother sheets to a work table during shearing or the like involvemechanically driven reciprocating means, usually spring-cushioned. Suchhold-down devices are simple, inexpensive and free of trouble, ascompared with conventional hydraulic hold-down devices presently knownand available. Also the energy put in to compress the springs inmechanical hold-downs is returned to the system on the up stroke.

7 Nevertheless,advantages are to be derived from the use ofhydraulically operated hold-down means, despite the fact that all theprior means oiiered to provide such a device have been subject tovarious objections, among them the cyclical action heretofore consideredinherent in the prololem, matters of objectionable leakage, and theamount of energy required to displace considerable quantities of liquidand develop the nose sary pressure therein.

According to the present invention hydraulic hold down means areprovided which provide a satisfactory and practical solution of thevarious problems which have heretofore militated against the successfuland practical employment of hydraulic pressure for hold-down purposes inplace of the usual mechanically driven spring cushioned hold-downs.

According to the present invention the hydraulic pressure generatingmeans is continuous in operation, not cyclical or intermittent as inprior proposals, and the arrangement is such that full hold-downpressure is applied almost instantaneously when desired, rather thanwith a material time lag. Further, the arrangement of the presentinvention is such that the work of operating the hold-down and providingholddown pressure is removed from the machine drive proper which reducesthe strength requirements of the machine and simplifies the constructionthereof.

The almost complete elimination of moving parts in the hold-downprovides a very quite operation and a small pump is adequate to providefull-hold-down pressure requirements. If desired the pump may be drivenfrom one of the gears in the conventional shear drive gear box insteadof by a separate electric motor, but in either event the advantage oflow power requirement is present. Further, the provision of hydraulicpressure means which are independent of the shear proper permits thehold-down. to be operated at will in motion, and this is of particularadvantage when cutting a sheet without using a fixed, positive sackgauge, as cutting along a line or to a measured dimension.

Since the main supporting bar for the hydraulically operated. hold-downfeet is stationary, it serves as a rigid structural tie between the endhousings of the shear and thus strengthens the machine and minimizes theneed of other crossbracing components between the end housings.

Of course, the provision of a stationary hold-down bar also eliminatesall of the usual guides and slide-hearing surfaces normally associatedwith mechanically reciprocating shear hold-downs and makes for a saferand less hazardous machine.

In the hold-down means of the present invention the hold-down feet arein contact with the work, resting thereon with the force of gravityonly, prior to the application of hydraulic holddown pressure. Thisprevents marring the surface of the sheet which often results whenvarious impact hold-down means are employed. This is of particularimportance when operating upon polished sheets of aluminum or stainlesssteel. This arrangement also eliminates the possibility of a workmanaccidentally getting his fingers under the hold-down feet.

Various embodiments of the principles of the present invention areillustrated in the accompanying drawings and described in detail in theensuing specification by way of example. However, many modifications maybe made within the spirit and scope of the invention and withoutdeparting from the principles thereof, the invention being limited onlyas defined in the appended claims.

In the drawings:

Fig. l isa front elevational view of a power shear showing one form ofhold-down bar and hydraulic cylinder means for-holding a sheet to thework table of the shear;

Fig. 2 is a general schematic View ofthe hydraulic circuit in one formof the present invention Fig. 3 is anether general schematic View ofanother adaptation of the hydraulic circuit of the present. inventionhaving a modified form of control means;

Fig. 4 is a fragmentary elevational view of a hcld down bar providedwith one form of hydraulic hold-down means;

Fig. 5 is a cross-sectional view taken on the line without setting theshearing or other mechanism 55 V-V of Fig. 4;

Fig. 6 is a cross-sectional view taken on the line VIVI of Fig. 4;

Fig. 7 is a view similar to Fig. 4 but showing another form of hydraulichold-down means;

Fig. 8 is a cross-sectional view taken on the line VIIIVIH of Fi '7;

Fig. 9 is a cross-sectional view taken on the line IXIX of Fig. '7;

Fig. 10 is another view similar to Fig. 4 but showing still another formof hydraulic holddown means;

Fig. 11 is a cross-sectional view taken on the line XI-XI of Fig. 10;and

Fig. 12 is a cross-sectional view taken on the line XII,XII of Fig. 10.

Throughout the several figures of the drawings like characters ofreference denote like parts and, referring to Fig. l, the power squaringshear there shown comprises a pair of end housing members I5 and I6which support therebetween a bed H having a work receiving table l8.

The shear includes a main drive shaft l9 having eccentric or crankconnection with the crosshead in conventional manner, the lattercarryinga shear blade 2|. In Fig. 1 the numeral 24 designates ahold-down bar which is rigidly fixed at its opposite ends to endhousings |5 and i6 as at 25 and 26. Hold-down bar 24 includes a seriesof hydraulic cylinders 21, each of which has a work clamp foot 28 at itslower end.

The details of construction and operation of the hydraulic cylinders andtheir associated hold-down feet will be described in detail later hereinin several forms. Fig. 1 shows schematically the hold-down bar 24 andhydraulic cylinders 21. In Fig. 2, a liquid reservoir or supply isindicated schematically at 30V and a continuous operating pump, whichmay be of relatively low displacement, has its intake port in freecommunication with the interior of the reservoir as by means of conduit32. Pump 3| has an outlet conduit 33 which leads to a branch conduit 34leading back to the reservoir and also a branch conduit 35 leading tohold-down bar 24.

At hold-down bar 24 branch conduit 35 communicates with a fluid passage36 which is associated with the hold-down bar and communicates freelywith all of the hydraulic cylinders 21 by means of branch passages 39.Another branch conduit 40 from conduit 35 leads back to the interior ofreservoir 30 and includes a manually operable valve 42 and a secondcontrol valve 43. Control valve 43, which is normally open, may beconveniently attached to the frame of the shear as shown in Fig. l andincludes a valve closing element 44 adapted to be actuated by a cam 45located upon drive shaft I9. Manual valve 42 is selectively closable tooperate the hold-down independently of the shear crosshead for placingwork to be cut to a line or to a measured dimenmen.

In Fig. 2 the numeral designates a pressure limiting valve whichdetermines the maximum pressure that pump 3| delivers to conduit 35.Valve 50 normally prevents flow into conduit 34 from conduit 33 but anyexcessive pressure in conduit 33 causes pressure limiting valve 50 toopen and relieve the excess pressure by exhausting through conduit 34back to the reservoir. Passage 36 of hold-down bar 24 is provided, atthe end opposite the connection with conduit 35, with a capillary tube52 or any tube having a very small inside diameter lead-ing therefromback to reservoir 30. When the shear is in condition for operation, butis not executing a shearing stroke,

valves 42 and 43 are open, and the liquid displaced by pump 3| andflowing through conduit 35 bypasses back to the reservoir throughpassage 43, at the same time maintaining the passage 36 and hydrauliccylinders 21 full of liquid.

Attention will now be given to Figs. 4, 5, and 6, which show oneembodiment of hold-down bar 24, hydraulic cylinders 21 and hold-downfeet 28. In Fig. 4 the hold-down bar 24 comprises upper and lower barportions 53 and 5| which are welded into an integral unit as at 63 and64. Beforewelding, the upper surface of bar 6| is milled or otherwisemachined to provide a groove 65 extending from near one end of bar 6| tothe other, stopping just short of the end of the bar in each instance.When bars 60 and 6| are joined by welding as just described, groove 65cooperates with the under side of bar 60 to form the liquid passage 36of Fig. 2. Communicating with this passage is an entrance passage 61which is connected to conduits 35 and 40.

In the schematic view, Fig. 2, conduit 36 is shown as declining to theleft as there viewed. In Fig. 4 the same substantive effect is attainedby disposing passage 61 at a higher elevation than passage 36. By reasonof this arrangement passage 36 will be maintained full of liquid by pump3| even though bypass 4|) is freely open to bypass excessive liquid backto reservoir 30. This also maintains the several hydraulic cylinders 21in a filled condition.

In the form shown in Figs. 4, 5, and 6, each hydraulic cylinder 21 has athreaded upper end 13 screwed into the under side of bar 6| and suitablypacked as indicated at 1|. Hydraulic cylinder 2'! has a fluid chamber 12and a passage !3 leading upwardly therefrom in registry with a passage15 which is formed in bar 5| and extends downwardly from groove 65,there being a separate passage 15 in the hold-down bar 24 for eachhold-down cylinder 21. Thus, passage 35 is in free communication withthe fluid chamber I2 in each of the hydraulic cylinders 21. Each of thehold-down feet 28 has an upper piston portion I? fixed thereto anddisposed for vertical sliding movement in fluid chamber 12 of cylinder21.

In the form shown in Figs. 4 through 6, the hold-down foot proper isgenerally rectangular, as viewed in plan and the bottom of cylinder 21is bifurcated as at to receive hold-down foot 28 and hold the sameagainst rotation. The forward lower edge of each hold-down foot 23 isrounded or beveled as indicated at 8| to facilitate moving a metal sheetor the like thereunder as at 83 in Figs. 4 and 5. The hold-down feet 28are normally in a lowered position against the work table I8 but underno pressure, resting there by gravity alone, so that a sheet may simplybe pushed beneath the several hold-down feet, whereupon they moveupwardly a distance equal to the thickness of the sheet and rest lightlyon the upper surface of the sheet, the oil from the cylinder beingdisplaced by way of passage 40.

As shown in Fig. 5, the inner wall of cylinder 2! has an encircling oilcollecting groove 85 and passages 81 and 88 lead therefrom to a leakagecollecting conduit 89 which may communicate in common with all of theseveral hydraulic cylinders and have gravity flow back to reservoir 30at 93 in Fig. 2.

As the crosshead 20 begins to descend in commencing a shearing stroke,cam 45 engages operator 44 to close the normally open hold-downoperating bypass valve 43. Since pump 3| is in continuous operation andthe passage 36 and the passages leading therefrom are all in aliquidfilled condition, pressure against hold-down feet 28. through themedium of pistons 11 is substantially immediate and direct. There is nonecessity for any movement of the piston 11 in cylinders 21 andanimmediate and secure hold down action results.

Each piston 11 fits rather closely in cylinder 21 both above and belowthe oil collecting groove 86. Accordingly, such liquid as passesdownwardly about the piston to the oil collecting groove, because of theconsiderable pressure in chamber 12 during a hold-down operation, flowsinto the substantially pressureless collecting groove and thenceoutwardly through passages 31 and 88 to leakage conduit 89 and back tothe reservoir. Since it requires a considerable pressure to move liquidpast piston 11 at any point along cylinder 21.; there will besubstantially no tendency for liquid to pass below the collecting groove86.

Because there is only the minute axial movement of piston 11 in cylinder21 equal to the thickness of the metal being sheared, there isrelatively no tendency for a iilm of oil to be continuously moveddownwardly between piston I1 and the interior wall of cylinder 21, asmight be the case if sliding movement of greater magnitude werenec'essary in operating the hold-down.

The relatively minute but substantially continuous flow of liquid fromthe left-hand end of passage 36 through capillary tube 52 wheneverpassage 36 is under-pressure carries off any air which may be entrainedwith the liquid in passage 35.

Reference will now be had to the modified hydraulic hold-down systemshown schematically in Fig; 3. In this form of the invention, as in thecase of the embodiment of Fig. 2, there is a stationary hold-down bar 95having a plurality of hydraulic hold-down cylinders 96 and a ccmmonpressure supply passage 61. In Fig. 3 the pressure generating pump isdesignated I613 and its outlet passage IIH is connected with a conduitI62 leading to the cylinder supply passage 91.

Interposed between the outlet passage IDI and conduit IE2 is a pressurelimiting valve I63 with aconduit I65 for returning liquid to a reservoirI65 when the pressure output of pump I66 exceeds the pressure setting ofvalve ms. The intake jconduit of pump I60 is designated I51 and leadsfrom reservoir I65.

As in the previous instance, the pressure supply passage '91 of thehydraulic cylinders terminates in communication with a capillary tube II6 which leads'back to reservoir I05 in the same manner and for the samepurpose set forth in connection with the description of Fig. 2. In Fig.3 the leakagereturn conduit similar to that designated 89 in Fig. 2 isdesignated I I I.

In addition to pressure passage 91 for the hydraulic cylinders, conduitI62 communicates with a bypass conduit I I2 which leads back toreservoir I05 and has interposed therein a solenoid controlled valve Ii3which is normally open and is closa-ble upon energization of anelectromagnet H4. In this form of the invention the single valve H3 isoperated either manually or automatically. Manual operation is providedfor operating the hold-down independently of the shear crosshead andautomatic operation accompanics reciprocating movement of the shearcrosshead.

A supply circuit for energizing electromagnet l'l is designated I I5 andhas connected in parallel therein'a manual: normally open treadleoperated switch H6 and a second normally open switch II! which replacesthe valve 43 in Fig. 1 so that cam 44' of the drive shaft of the machineacts upon an operator H8 of the switch to automatically close the samefor predetermined periods during the reciprocation of the shearcrosshead. It is believed that the operation of the electricallycontrolled hydraulic hold-down system of the embodiment of Fig. 3 willbe clear from the foregoing.

A second embodiment of the hold-down bar and hydraulic cylinder meansper se is illustrated. in Figs. 7, 8, and 9. In this formthe pres sureand leakage conduits for the cylinders are independent of the hold-downbar proper and the entire cylinder and conduit assembly is removablefrom the stationary hold-down bar.

Referring to Figs. 7 and 8 the numeral I20 designates a hold-down barsimilar to the bar 24 of Figs. 1, 2, 4 and 5 and one of a series ofhydraulic cylinder elements is designated I21. As in the precedinginstance, cylinder I2I has a pressure chamber I22, a piston I23vertically slidable therein, and an oil collecting groove I24 about itsinner periphery. Piston I23 is provided at its lower end with ahold-down foot I26 similar to that previously described and the lowerportion of cylinder IZI is bifurcated as at I28 to guide hold-down footI26 against rotative movement.

Cylinder I2I is secured against the under side of hold-down bar I26 bymeans of screws I36 which extend upwardly through lugs I3! which projectfrom opposite sides of cylinder I2I.

Cylinder I2! has a forwardly offset upper extension I34 and the severalupward extensions I34 of a series of hydraulic cylinders are connectedby pressure conduits I35 and leakage return conduits I36. Pressureconduits I35 apply operating pressure to pressure chamber I22 by meansof passages I31, I38, and I39 formed in each of the upward extensionsI34 of the hydraulic cylinders I2 I. As appears best in Fig. '7, theliquid collecting groove I24 communicates with leakage return conduitsI36 by means oi passages I II, I42, and I43 formed in the body portionof cylinder I2I and upward extension I34.

In the further embodiment of the hydraulic cylinder means per seillustrated in Figs. 10

through 12, the numeral I designates one of a series of hydrauliccylinders having lateral lugs I5I at its upper end for securement to theunder side of a hold-down bar I52 by means of screws I53.

Hydraulic cylinder I50 has a pressure chamber for receiving a piston I55and the latter has a hold-down foot I51 at its lower end as in the.

previously described embodiments. Cylinder I56 is bifurcated at itslower end as at I58 to prevent rotation of the hold-down foot. The innerperiphery of the pressure chamber I55 of cylinder I56 is provided with aleakage collecting groove I66 as in the previously describedembodiments.

A guard plate I depends from a bar [16 to which it is fixed by weldingor the like and bar it is secured to hold-down bar I52 by means ofscrews I11. Guard plate 115 extends downwardly behind cylinders I50 to apoint just above work table [8 to prevent an operator from reachingunder the hold-down bar between a pair of spaced cylinders I50. Similarguard plates are intended to be employed in conjunction with thepreviously described hydraulic cylinder embodiments, the guard plate ofthe first described embodiment being designated 80 in Fig. 4.

The operation of the several forms of hydraulic cylinder arrangementsillustrated herein is the same in function and theory as describedgenerally in connection with the schematic hydraulic circuit, Fig. 2,and obviously any of the cylinder arrangements of Figs. 4 through 12 maybe used in the systems of either Figs. 2 or 3.

What claimed is:

1. A hold-down for shears like machines, said hold-down comprising arigid support, hydraulic pressure actuated hold-down means supportedthereby, a pressure conduit for energizing said pressure actuatedhold-down means by application of hydraulic pressure thereto, acontinuously operating pressure generator in free continuouscommunication with said pressure conduit, a normally open relief conduitbetween said pressure generator and said pressure conduit to relieve thepressure conduit, valve means in said relief conduit closable to preventfiow therein and thus cause the pressure generator to act directlyagainst the pressure conduit to activate the hold-down means, and meansoperable in timed relation with the operation of the machine proper forautomatically closing and opening said valve at predetermined points inthe cycle of operation of the machine.

2. A hold-down for shears and like machines, said hold-down comprising arigid support, hydraulic pressure actuated hold-down means supportedthereby, a pressure conduit for ener izing said pressure actuatedhold-down means by application of hydraulic pressure thereto, acontinuously operating pressure generator in free continuous coim;unication with said pressure conduit, a normally open relief conduitbetween said pressure generator and said pressure conduit to relieve thepressure conduit, and valve means in said relief conduit closable toprevent flow therein and thus cause the pressure generator to actdirectly against the pressure conduit to activate the hold-down means,means operable in timed relation with the machine proper forautomatically closing opening said valve at predetermined points in thecycle of operation of the machine, and manual means for selectivelyclosing said relief conduit to actuate the hold-down means independentlyof machine operation.

3. A hold-down for shears and like machines, said hold-down comprising arigid support, a hydraulic pressure actuated hold-down means s1; portedthereby, a pressure conduit for energizing pressure actuated hold-downmeans by application of hydraulic pressure thereto, a continuouslyoperating pressure generator in free continuous communication with saidpressure conduit, a normally open relief conduit in open communicationwith said pressure conduit to relieve the pressure conduit, and meansfor closing said relief conduit automatically for operating saidhold-down in timed relation with the operation of the machine proper andmanually for operating the hold-down independently of machine operation.

4. A hold-down for shears and like machines, said hold-down comprising arigid support, hydraulic pressure actuated hold-down means supportedthereby, a pressure conduit for energizing said pressure actuatedhold-down means by application of hydraulic pressure thereto, acontinuously operating pressure generator in free continuouscommunication with said pressure conduit, a normally open relief conduitin open communication with said pressure conduit to relieve the pressureconduit, means for closing said relief conduit automatically in timedrelation with the operation of the machine proper to activate thehold-down means, and means for closing the relief conduit by manualoperation to activate the hold-down means independently.

5. A hold-down for shears and like machines, said hold-down comprising arigid support, bydraulic pressure actuated hold-down means supportedthereby, a pressure conduit for energizing said pressure actuatedhold-down means by application of hydraulic pressure thereto, acontinuously operating pressure generator in free continuouscommunication with said pressure conduit, a normally open relief conduitbetween said pressure generator and said pressure conduit to relieve thepressure conduit, and valve means in said relief conduit automaticallyclosable in timed relation with the operation of the machine proper tocause the pressure generator to act directly against the pressureconduit to activate the hold-down means.

6. A hold-down for shears and like machines, said hold-down comprising arigid support, hydraulic pressure actuated hold-down means supportedthereby, a pressure conduit for energizing said pressure actuatedhold-down means by application of hydraulic pressure thereto, acontinuously operating pressure generator in free continuouscommunication with said pressure conduit, a normally open relief conduitbetween said pressure generator and said pressure conduit to relieve thepressure conduit, and valve means in said relief conduit automaticallyclosable in timed relation with the operation of the machine proper tocause the pressure generator to act directly against the pressureconduit to activate the hold-down means, said valve means being manuallyclosable to activate the holddown means independently of the operationof the machine proper.

'7. A hold-down for shears'and like machines, said hold-down comprisinga rigid support, hydraulic pressure actuated hold-down means supportedthereby, a pressure conduit for energizing said pressure actuatedhold-down means by application of hydraulic pressure thereto, acontinuously operating pressure generator in free continuouscommunication with. said pressure conduit, a normally open reliefconduit between pressure generator and said pressure conduit to relievethe pressure conduit, valve means in said relief timed relation with theoperation of the machine proper to cause the pressure generator to actdirectly against the pressure conduit to activate the hold-down means,and other valve means in said relief conduit selectively closable toactivate the hoiddown means independently of the operation of themachine proper.

8. A hold-down f shears and like machines, said hold-down comprising arigid support, hydraulic pressure actuated hold-down means conduitautomatically closable in 9 supported thereby, a pressure conduit forenergizing said pressure actuated hold-down means by application ofhydraulic pressure thereto, a continuously operating pressure generatorin free continuous communication with said pressure conduit, a normallyopen relief conduit between said pressure generator and said pressureconduit to relieve the pressure conduit, and a normally open valve insaid relief conduit, means for automatically closing said valve in timedrelation with the operation of the machine proper to cause the pressuregenerator to act directly against the pressure conduit to activate theholddown means, and other means manually operable for closing said'valveto activate the holddown means independently 40f machine operation.

9. Hold-down means for shears and like machines, said hold-down meanscomprising a rigid support, a plurality of hydraulic hold-down cylinderssupported thereby, piston means in said cylinders adapted to be urgeddownwardly against a work piece upon application of hydrau lic pressureto said cylinders, a common pressure conduit for said cylinders, acontinuously opcrating pressure generator in free continuouscommunication with said common pressure iconduit, a relief conduitbetween said pressure generator and said common pressure conduit torelieve the pressure conduit, and valve means in said relief conduitclosable to prevent bypass of said pressure conduit and thus cause thepressure generator to act directly against the common pressure conduit.

10. A hold-down for shears and like machines, said hold-down comprisinga ri id support, .a plurality of hydraulic pressure actuatedv holddownmeans supported thereby, a common pressure conduit for energizing-saidpressure actuated hold-.downmeans, a continuously operating'pressuregenerator in free continuous communication with said common pressureconduit, a relief conduit between said pressure generator and saidcommon pressure conduit'to relieve the pressure conduit, and valve meansin said relief conduit closable to prevent bypass of said'pressureconduit and thus cause the pressure generator to act directly againstthe common pressure conduit.

ll. Hold-down means for shears and like machines, said hold-down meanscomprising a rigid support, a plurality of pairs of piston and cylinderelements including fluid pressure chambers, one of each of said pairs ofelements being fixed to the support and the other normally resting onthe work table under the force of gravity whereby a work piece may heslipped thereunder, a common pressure conduit in communication with theseveral fluid pressure chambers, a continuously operating pressuregenerator in free continuous communication with said common pre sureconduit, a relief conduit between said pressure generator and saidcommon pressure conduit to relieve the pressure conduit, and valve meansin said relief conduit closable to prevent bypass of said commonpressure conduit and thus cause the pressure generator to act directlyagainst the common pressure conduit and the several fluid pressurechambers and thus cause said other of the pairs of piston and cylinderelements to be urged downwardly against the work piece.

12. Hold-down means for shears and like machines, said hold-down meanscomprising a rigid support, a plurality of pairs of piston andcylinderelements, said cylinder elements being fixed to the support and having"fluid pressure chambers, the piston elements normally resting .on thework table under the force of gravity whereby a work piece may beslipped thereunder, a common pressure-conduit in communication with theseveral fluid pressure chambers, a continuously operating pressuregenerator in free continuous communication with said common pressureconduit, a relief conduit between said pressure generator and saidcommon pressure con duit to relieve the pressure conduit, and valvemeans in said relief conduit .closable to prevent bypass ofsaidcommonpressure conduit and thus cause the pressure generator to actdirectly against the common pressure conduit and the several Jfiuidpressure chambers and thus cause said other of the pairs .of piston andcylinder elements to be urged downwardly against the Work piece;

13. Hold-down means for shears and lilzelmachines, said hold-down meanscomprising a rigid support, 'a pair of :piston and cylinder elementsincluding a fluid pressure chamber, one of said pair of elements beingfixed to the support and the other normally resting on the work tableunder the force of gravity whereby a workpiece may be slippedthereunder, a continuously operating pressure generator in freecontinuous communication with said pressure chamber, a relief conduitbetween said pressure generator and said pressure chamber to relieve thepressure chamber, and valve means in said relief conduit closable toprevent bypass :of said pressure chamber and thus cause the pressuregenorator to act directly against said other of the pair of piston andcylinder elements to urge the same downwardly against :the workpiece.

1,4. Hold-downmeansfor shears and like me.- chines, said hold-down meanscomprising a rigid support, a pair of piston andcylinoler elements,saidcylinder elements beingfixed to the support and having a fluidpressure chamber and the piston element normally resting on the worktable under thefo-rce of gravity whereby a work piece may be slippedthereunder, a continuously operating pressure generator in :freecontinuous communication with said fluid pressure chamber, :a reliefconduit between said pressure generator and said pressure chamber 'torelieve the pressure chamber, and valve means in said reliefconduitclosable to prevent bypass of said pressure chamber and. thuscause the pressure generator to act directly against the piston elementand thus cause said piston element to be urged downwardly against thework piece.

15. A hold-down for shears and like machines, said hold-down comprisinga rigid support, hydraulic pressure actuated hold-down means supportedthereby, a pressure conduit for energizing said pressure actuatedhold-down means by application of hydraulic pressure thereto, acontinuously operating pressure generator in free continuouscommunication with said pressure conduit, a normally open relief conduitbetween said pressure generator and said pressure conduit leadingtherefrom at an elevated point relative to the pressure conduit torelieve the pressure conduit of iiuid pressure without permitting liquiddrainage therefrom, and valve means in said relief conduit closable tocause the pressure generator to act directly against the pressureconduit to activate the hold-down means.

16. A hold-down for shears and like machines, said hold-down comprisinga rigid support, hydraulic pressure actuated hold-down means supportedthereby, a pressure conduit for energizing said pressure actuatedhold-down means by application of hydraulicpressure thereto, acontinuously operating pressure generator in free continuouscommunication with said pressure conduit, a normally open relief conduitbetween said pressure generator and said pressure conduit to relieve thepressure conduit, valve means in said relief conduit automaticallyclosable in timed relation with the operation of the machine proper tocause the pressure generator to act directly against the pressureconduit to activate the hold-down means, and a capillary tube leadingfrom said pressure conduit beyond said hydraulic pressure actuatedhold-down means to carry off air which may be entrained with thehydraulic medium in said pressure conduit.

'17. A hold-down for shears and like machines, said hold-down comprisinga rigid support, hydraulic pressure actuated hold-down means supportedthereby, a pressure conduit for energizing said pressure actuatedhold-down means by application of hydraulic pressure thereto, acontinuously operating pressure generator in free continuouscommunication with said pressure conduit, a normally open relief conduitbetween said pressure generator and said pressure conduit leadingtherefrom at an elevated. point relative to the pressure conduit torelieve the pressure conduit of fluid pressure without permitting liquiddrainage therefrom, valve means in said relief conduit closable to causethe pressure gen erator to act directly against the pressure conduit toactivate the hold-down means, and a capil lary tube leading from saidpressure conduit beyond said hydraulic pressure actuated hold-down meansto carry on air which may be entrained with the hydraulic medium in saidpressure conduit.

18. Hold-down means for shears and like machines, said hold-down meanscomprising a rigid support, a pair of inter-sliding piston and cylinderelements including a fluid pressure chamber, one of said pair ofelements being fixed to the support and the other normally resting onthe work table under the force of gravity whereby a work piece may beslipped thereunder, the fixed element having an encircling groove at itspoint of inter-sliding engagement with said other element, and a conduitfor draining leakage liquid from said groove, a continuously operatingpressure generator in free continuous communication with said fluidpressure chamber, a relief conduit between said pressure generator andsaid pressure chamber to relieve the pressure chamber, and valve meansin said relief conduit closable to prevent bypass of said fluid pressurechamber and thus cause the pressure generator to act directly againstsaid other of the pair of piston and cylinder elements to urge the samedownwardly against the work piece.

19. Hold-down means for shears and like machines, said hold-down meanscomprising a rigid support, a plurality of hydraulic hold-down cylinderssupported thereby, piston means in said cylinders adapted to be urgeddownwardly against a work piece upon application of hydraulic pressureto said cylinders, a common pressure conduit for said cylinders, acontinuously operating pressure generator in free continuouscommunication with said common pressure conduit, a bypass conduitbetween said pressure generator and said common pressure conduit torelieve the pressure conduit, and valve means in said bypass conduitclosable to prevent bypass and thus cause the pressure generator to actdirectly against the common pressure conduit, said cylinders havinginternal peripheral grooves adjacent to said piston means for collectingliquid leaking past said piston means, and a common conduit for drainingleakage liquid from said grooves.

20. A hold-down for shears and like machines, said hold-down comprisinga rigid support, a plurality of hydraulic pressure actuated hold-downmeans supported thereby, a common pressure conduit for energizing saidpressure actuated hold-down means, a continuously operating pressuregenerator in free continuous communication with said common pressureconduit, a relief conduit and valve means in said relief valve conduitclosable to cause the pressure in said pressure conduit from saidgenerator to act directly against said hydraulic pressure actuatedhold-down means.

FREDERICK E. MUNSCHAUER.

REFERENCES CITED The following references are of record in the file ofthis patent:

UNITED STATES PATENTS Number Name Date 616,801 McNutt Dec. 27, 18982,323,770 Hazelton July 6, 1943

